In the context of digital circuits, noise is defined as any deviation of a signal from its stable value in those subintervals of time when it should otherwise be stable. Noise in digital circuits can be attributed to several sources such as leakage noise, charge-sharing noise, cross-talk noise, power supply noise, shot noise, thermal noise, and flicker noise. Rigorous noise analysis and noise considerations during design are becoming increasingly important.
The current capability of a MOSFET (Metal Oxide Semiconductor Field, Effect Transistor) is inversely proportional to a MOSFET's length. As a result, more current may be sourced by an individual MOSFET by reducing the length of the MOSFET. However, as the length of a MOSFET is reduced, other problems are created. For example, the threshold voltage may be lowered, resulting in higher levels of leakage current. In addition, leakage current of a MOSFET may introduce noise into a circuit by leaking charge from a node that would ideally retain its initial charge.
Charge-sharing may be used in a positive manner. DRAMs (Dynamic Random Access Memory) use the principle of charge-sharing to create dense memory devices. DRAMs store an individual data bit by either storing more charge on a capacitor or storing less charge on a capacitor. A DRAM reads data by measuring the change in voltage on a bitline when the charge on a capacitor is charge-shared with the charge on the bitline. However, unwanted charge-sharing between memory elements may cause correct data stored in a memory element to “flip” to incorrect data.
One embodiment of this invention reduces the likelihood that charge-sharing between multi-threaded memory cells will cause incorrect data to be stored in a memory cell. The implementation of this embodiment does not create additional drive fights when the multi-threaded memory cells are written. In addition, charge loss in multi-threaded memory devices due to leakage is reduced by an embodiment of the invention. A detailed description of one embodiment of this invention is described later.